1. Field of the Invention
This invention relates to a sulfuric acid concentration sensor enabling an easy measurement of the concentration of sulfuric acid and a lead acid battery equipped with such a sulfuric acid concentration sensor.
2. Description of the Prior Art
Sulfuric acid is used widely in industrial fields, and it is frequently required to know the concentration of sulfuric acid when using it. Accordingly, a measurement of the concentration of sulfuric acid is very significant matter from the industrial viewpoint. In a lead acid battery for example, its sulfuric acid concentration varies according to charging and discharging as expressed by the following equation (II), so that a charge/discharge quantity of a lead acid battery can be measured by measuring the sulfuric acid concentration. If the sulfuric acid concentration can be measured easily, the charge/discharge quantity of the lead acid battery can be controlled. Since the sulfuric acid concentration normally varies within a range of 8 to 45% in the charging and discharging of lead acid battery, it is generally said that the measurement in this range of concentration is specially important. ##STR1##
The large portion of lead acid batteries are used for starting automobiles. However, when the battery is in an insufficiently charged state, its residual capacity is not enough to carry out the starting operation. Since a driver has not been able to previously know such a defective state, he has encountered much troubles so far. If the charged state can be known in advance, countermeasures can be taken such as carrying out a supplementary charging before stopping the automobile. Therefore, there has been a strong demand for a lead acid battery equipped with a sensor capable of indicating a charge/discharge quantity. In existing generators, a charging current at the time of charge end is controlled by a charging voltage, however, a control suitable for the battery can not be executed because the charge voltage changes according to a temperature and history of battery etc. On the contrary, the specific gravity of the electrolyte, i.e., the sulfuric acid concentration, correctly indicates the charge/discharge quantity. Consequently, the charge end can be known exactly by the sulfuric acid concentration so that a charge control suitable for the battery can be executed. For this reason, there has been a strong demand for a lead acid battery equipped with a sensor capable of indicating a charge/discharge quantity on the basis of the sulfuric acid concentration.
As methods for measuring the sulfuric acid concentration, those shown by the following (1) to (4) have been known so far. They each include respective problems and are expensive in manufacturing cost, so that they have not been put in practical use for a sulfuric acid concentration sensor of lead acid battery for automobile.
(1) Refractive index measurement method
This is a measurement method utilizing a property of the sulfuric acid that its refractive index changes according to its concentration, and a measurement device is composed of a light emitting diode, a light receiving diode and an optical path. This method requires a comparatively long optical path in a sulfuric acid solution which is an object of measurement, and conversion of light into an electrical current, so that a compact and inexpensive sensor can not be fabricated. Although this sensor is now used in a stationary lead acid battery, it has not so far been put to practical use for an automobile lead acid battery.
(2) Specific gravity measurement method
This is the method for measuring a specific gravity of sulfuric acid utilizing a float, which is inexpensive and convenient, but is difficult in accomplishing an automatic reading. Further, this method includes various difficulties from the standpoints of manufacturing cost and structure in order to take in the specific gravity as an electrical signal into a control system.
(3) Electrochemical method
This is a measurement method in which a sensor electrode system comprising a metal, sulfuric acid and metal oxide is prepared separately, so as to measure the concentration by means of its dependency of electromotive force on the sulfuric acid concentration. An appropriate electrode system has not not found as of yet, and the existing system has the disadvantage that both electrodes are required to be subjected to a periodical renewal.
(4) Electric conductivity method
This is a method for measuring an electric conductivity of sulfuric acid. This method is simple at first thought. However, since the electric conductivity of sulfuric acid reaches its maximum value at its 30% concentration, the sulfuric acid concentration does not become a single-valued function of the electric conductivity (Issued by John Wiley & Sons Company, "Encyclopedia of Chemical Technology", second edition, vol. 19, page 446, 1969), so that the sulfuric acid concentration can not be calculated equivocally from the value of electric conductivity. For this reason, a complex data processing system is necessary which causes a high manufacturing cost.
On the other hand, a retainer type lead acid battery in which an electrolyte is impregnated in a glass mat, etc. has comes into wide use recently. The electrolyte has no flowability at all and the quantity of electrolyte is limited, so that no suspended liquid exists at all in this battery. Therefore, even the above method (1) can not be used.